This invention relates to a method for operating a boiling water-type atomic power plant (which will be hereinafter referred to as BWR) with a good effect on control of an increase in the dose rate of its primary cooling system, and particularly to a method for operation in view of quality of core water suitable for controlling a cobalt-60 concentration in core water, more particularly a dose rate of the primary cooling system by controlling the core water to be neutral or slightly alkaline at any time, thereby suppressing dissolution of cobalt-60 deposited on fuel rods.
Basically, BWR is characterized by direct cycle, where the steam generated in a nuclear reactor is directly fed to a turbine to generate electric power.
FIG. 1 shows the major units of BWR primary cooling system and flow in the cooling system, where the cooling water in nuclear reactor pressure vessel 7 is forced to circulate by means of recycle pump 2. The cooling water will be hereinafter referred to as core water. Steam generated in nuclear reactor 1 is fed to turbine 3 after moisture has been removed in a separator and a drier provided above the core. A portion of the steam is withdrawn from the turbine as a turbine bleed steam, and used as a heat source for high pressure and low pressure heaters 4, whereas most of the steam is condensed to water in condenser 5. The condensate is almost completely degasified in the condenser, and the oxygen and hydrogen generated by radiolysis of water in the core can be almost completely removed at the same time.
Generally, the condensate is heated to about 200.degree. C. in the low pressure and high pressure heaters arranged in stages, and again fed to the nuclear reactor. To suppress formation of radioactive corrosion products in the nuclear reactor, ion exchange resin filter 6 such as a desalter, etc. is provided between the condenser and the low pressure heater, and all the amount of the condensed water is treated therein, thereby mainly removing metallic impurities from the condensed water to maintain the condensed water at a high purity.
To reduce formation of metallic impurities due to corrosion of the materials of construction used in the primary cooling system, stainless steel is utilized in principle as the major material of construction. Nuclear reactor pressure vessel 7 made of carbon steel has a build-up welding of stainless steel on the inside surface to prevent the carbon steel from any direct contact with core water. In addition to the consideration of the materials of construction, a portion of core water is purified in core water purifying unit 8 to intensively remove a very small amount of metallic impurites formed in the core water.
In spite of these measures for the materials of construction and water quality control, the presence of a very small amount of metallic impurities in core water is inevitable, and particularly cobalt is radioactivated into cobalt-60 mainly on the surfaces of fuel rods and accumulates on the materials of construction in the primary cooling system to cause an increase in the dose rate in the primary cooling system.
In a pressurized water type atomic power plant (which will be hereinafter referred to as PWR) as another type of light water reactor, a chemical solution is injected into the core water to adjust the dissolved oxygen concentration and maintain pH at 9 or higher in the core water as the measures for controlling the corrosion of the materials of construction.
FIG. 2 shows the major units of PWR primary and secondary systems and its cooling system flow, where the heat generated in the core is transferred to the secondary system in steam generator 9 as a contact point of the primary system with the secondary system, and removed from the primary system. Since the primary system forms a completely closed loop, even the injection of a chemical solution into the primary system as mentioned above requires no discharge from the primary system, resulting in less consumption of the chemical solution. However, to suppress formation of tritium in the primary system, expensive .sup.7 LiOH.sup.- is used at a high concentration for pH adjustment. To reduce the amount of .sup.7 LiOH.sup.-, the capacity of purifying system for the primary system is made to 1/10 or less of that for BWR. As a result, no better removal effect on the formed metallic impurities is obtained, and the concentration of radioactive corrosion products in the primary system for PWR is higher than that for BWR owing to the overall poor control effects on formation and removal of metallic impurities.
When an attempt is made to control the corrosion of materials of construction in the primary cooling system in BWR by injection of a chemical solution as in PWR, the following three problems are encountered in the turbine system. That is, a portion of the injected chemical solution is entrained by steam, and it is necessary to inject a larger amount of the chemical solution to make up the loss of the chemical solution. This necessitates a larger capacity of the injection facility and preparation of a larger amount of the chemical solution. Attack of the chemical solution carried over by the steam to machine members such as turbine blades, etc. causes new damages. Furthermore, radioactivation of alkali metals, for example, into .sup.24 Na, etc. brings about radioactive contamination in the turbine system.
In BWR, the core water is thus controlled to neutral pure water without addition of the chemical solution, as mentioned above. When there is contamination of organic impurities, for example, ion exchange resin, pH is shifted to acidic side, bringing about an unfavorable atmosphere to the materials of construction.
As a result of extensive studies, the present inventors have found the following quite a new fact: when the pH of core water is shifted to the acidic side, dissolution of radioactive corrosion products deposited on the surfaces of fuel rods into core water is increased, and thus the concentration of radioactive corrosion products in the core water is increased, thereby causing accumulation of radioactive corrosion products in the units or piping in the primary cooling system and increasing the dose rate at the nuclear reactor shut-down.